Hey, welcome to the wonderf world of fantasy
I've got some advice on the budget part firstly,
1: the warsphinx is an amazing unit, model it with the tomb guard crew. The box acctually gives you a base for the tomb king so he's pretty much free. So 60$= 240 point monster + tomb king
2:buy the sepulchral stalker/necropolis knights kit. Build the sepulchral stalkers. (There just awesome anyways) but the kit then gives you a LOT of extra bits that will allow you to make impressive looking heroes/lords.
3: buy the empire battle wizards box. You get 2 guys that with very little conversion ( using bits from the previously mentioned box) can become liche priests.)
Now that I've given my two warp tokens on budget, here they are on tactics.
1: the skeleton horse archers are absolutely great, take a unit of 10-12, and then they scout. So they become a very solid unit for war machine hunting.
2: chariots are great, but you have to utilize the "and the tomb kings rode to war" if you want them to be more than a supporting unit. Take anywhere from 6-9. Your doing a lot of strength 5-6 impact hits. If your just using them as supporting units. Do squads of 3, and make sure they flank charge
3: a squad of simple skeleton warriors with a tomb king, and you have a squad of guys at
WS 6. It's downright amazing.
4: tomb herald
bsb's are good with standard of the undying legion. Put him and your king in a unit of skeletons or tomb guard and you have a high weapon skill regenerating squad of pain.
There's my warp tokens, hope it helps!